Adult Health · 14 min read
ABG Interpretation & Acid-Base Balance
A repeatable four-step read that turns any arterial blood gas into an answer.
The four-step read
First, look at the pH. Below 7.35 is acidosis, above 7.45 is alkalosis. Second, look at the PaCO2 — this is the respiratory value, and it moves opposite to pH when the lungs are the problem. Third, look at the bicarbonate — the metabolic value, which moves in the same direction as pH when the kidneys or metabolism are the problem. Fourth, decide whether the other system has begun to compensate.
Uncompensated means the opposing value is still normal. Partially compensated means it has shifted but the pH is still abnormal. Fully compensated means the pH has returned into range while both other values remain abnormal.
Common clinical causes
Respiratory acidosis is hypoventilation: opioid oversedation, COPD exacerbation, chest trauma, neuromuscular weakness. Respiratory alkalosis is hyperventilation: anxiety, pain, early sepsis, high altitude, pulmonary embolism.
Metabolic acidosis comes from acid gain or bicarbonate loss: diabetic ketoacidosis, lactic acidosis from shock, kidney failure, prolonged diarrhea. Metabolic alkalosis comes from acid loss or base gain: prolonged vomiting, nasogastric suction, excessive antacids, diuretics.
Test pattern
Vomiting and nasogastric suction remove hydrochloric acid, so they cause metabolic alkalosis. Diarrhea loses bicarbonate from below, so it causes metabolic acidosis. Remember which end.
What the nurse actually does
For respiratory acidosis the intervention is always about moving air: reposition to high Fowler's, encourage deep breathing and coughing, suction if secretions are the obstruction, and anticipate reversal agents or ventilatory support.
For metabolic acidosis the intervention treats the source — insulin and fluids for ketoacidosis, perfusion restoration for lactic acidosis, dialysis for kidney failure.
Next lesson
Fluids, Electrolytes & IV Tonicity